1. Field of the Invention
The present invention relates to a bump forming method and system, and in particular, relates to a system for forming bumps on a semiconductor wafer such as a silicon wafer or a semiconductor chip such as a silicon chip, with high positional accuracy.
Priority is claimed on Japanese Patent Application No. 2004-126872, filed Apr. 22, 2004, the content of which is incorporated herein by reference.
2. Description of the Related Art
In conventional semiconductor devices such as ICs, LSIs, and VLSIs, bumps are formed at specific positions (typically, on pads) on a silicon wafer by printing, ball-bump forming, plating, transfer, wire-bonding, or the like. For example, in transfer, an attracting plate is used for attracting solder or gold balls, and the attracted balls are compressed onto pads on a silicon wafer, so as to form bumps on the pad (see Japanese Unexamined Patent Application, First Publication No. 2003-258012).
FIG. 8 is a diagram showing a concrete example of a bump forming method using a conventional attracting method. In FIG. 8, reference numeral 21 indicates a silicon wafer, reference numeral 22 indicates a stage on which the silicon wafer 21 is placed; reference numeral 23 indicates a chuck (i.e., a fastening device) for fastening the silicon wafer 21 onto the stage 22, reference numeral 24 indicates gold balls (i.e., bump materials), reference numeral 25 indicates an attracting and compressing plate (i.e., an attracting and compressing device) for performing vacuum attraction and ultrasonic compression of the gold balls 24, and reference numeral 26 indicates a finishing plate for pressing the gold balls 24 so as to provide a specific shape to each ball.
The structure of the attracting and compressing plate 25 will be explained with reference to FIG. 9. As shown in the figure, a panel plate 31 has a flat plane 31a (which is a major plane) on which concave portions 32, each having an approximately hemispherical shape, for attracting and holding the gold balls 24 are formed, where the positions of the concave portions 32 respectively correspond to the positions of the pads formed on the silicon wafer 21. A passage 33, an end of which is connected to each concave portion 32, is formed through the plate 31, where the other end of passage is connected to a vacuum pump or the like, which is used in vacuum attraction of the gold balls 24.
When forming gold balls 24 on the pads on the silicon wafer 21, first, the attracting and compressing plate 25 is brought close to the gold balls 24, and the inside of each concave portion 32 is decompressed via the passage 33 by using a vacuum pump or the like, so that each gold ball 24 is attracted into the concave portion 32. In the next step, the attracting and compressing plate 25 is moved above the silicon wafer 21 so that the plate 31 is brought close to the silicon wafer 21. As a result, each gold ball 24, which has been attracted to the concave portion of the attracting and compressing plate 25, is placed onto the silicon wafer 21. In the following step, ultrasonic waves having a specific frequency are applied to the plate 31 so as to vibrate each gold ball 24 and directly compress and bond the gold ball onto the aluminum pad, thereby producing a bump.
Recently, the pitch between pads on a semiconductor device is made shorter, that is, recent devices have fine pitches, so that micro balls to be attracted onto such pads are required. However, in a conventional bump forming method by attraction, the attraction process may repeatedly fail, thereby degrading the yield rate. More specifically, two or three balls 24 may be attracted into a single concave portion 32, or some balls 24 may fall from the concave portions 32; therefore, the success rate of transfer of balls 24 may be reduced and the yield rate of products may also be reduced.
In addition, micro balls 24 tend to fly in air flow, like powder, and also tend to attach to each other by static electricity; thus, it is very difficult to handle them.